1. Field of the Invention
The invention is directed to the fuses used in integrated circuits made by MOS technology.
2. Description of the Prior Art
Fuses are used in such circuits to configure them, namely to define, at a given moment, their functions, their resources and their modes or paths of access to these sources. For example, in memory cards, fuses such as these enable an initial EPROM or EEPROM memory plane to be partitioned into zones that are accessible in reading/writing mode and others that are accessible in reading mode only.
Several successive configurations are sometimes necessary. Each of these corresponds to a particular level of use, this use being more restricted and more specialized at each new configuration. For example, it is possible to define three levels or modes of use for a chip card or memory card. Since the first mode is the one obtained after the manufacture of the card, which is called the factory mode, it is the most open and least specialized one. It is characterized by few restrictions or prohibitions with respect to access path. The chip cards may be used for a variety of applications such as telephone, banking and other applications. Hence a first configuration is made to give the card a particular feature: for example the card in factory mode is configured for a banking application. It may then be supplied to a bank. This is an intermediate mode. The access functions and modes have been restricted, i.e. configured to correspond to the formalities proper to a bank card as laid down in a set of specifications of the bank. The bank may then program its own secret information such as an identification number, account number, permitted operations etc. in a given zone. It should then bar any subsequent reprogramming of this zone. It therefore carries out a second configuration, which takes the card in intermediate mode into a final, user mode. It can then forward the card to a user.
This non-exhaustive example of use, however, testifies not only to the usefulness of the fuses in enabling configuration but also to the reliability and security that should be given to these fuses to ensure that the configurations (successive otherwise) of an integrated circuit are irreversible: a set of functions cannot be modified in any way. Indeed, to give only one example, it should not be possible for the user to modify the secret information.
There are different known fuses which can be used in integrated circuits. These fuses are made, for example, in the form of a very thin layer of polycrystalline silicon and are supplied with current, for example by a transistor. When a current flows for a certain time, a local heating occurs because of the very thin section of the fuse. This heating is enough to volatilize the metal: the fuse fuses or breaks down. In fact, it sometimes happens that the fuse does not break down. In practice, a fuse such as this is not reliable.
Besides, it is necessary to note the very widespread use of EPROMs and, above all, EEPROMs in chip cards. The need to keep a record of certain pieces of information and of operations in a chip card which is sometimes under voltage (in the reader) and sometimes off voltage, has ruled out the use of RAMs which are volatile and ROMs which are non-volatile but programmable by mask, during manufacture.
The fact that a permanent record has to be kept, not of all the operations but of only some of them such as the balance of the account for an banking application, only between one use and another of the chip card, has given a certain advantage to the EEPROM, which is electrically erasable in the reader. The fact that an EPROM cannot be erased on the site of the reader leads to the consumption of all the programmable memory. This makes it necessary to change the card when this memory has been entirely programmed. Now, while the notion of a disposable card is still in vogue, the notion of a card rechargeable on site is preferred to it, especially for simple economic reasons. An advantage of a fuse according to the invention is that it forms part of an EEPROM memory plane such as this.